Single-Cell Proteins (SCPs) from Methanol

2.4. Single-Cell Proteins (SCPs) from Methanol

The terminology single-cell protein (SCP) was first and foremost coined by some researchers at the famous MIT-Massachusetts Institute of Technology (USA) in the year 1966. As to date, SCP, covers

a rather broad spectrum coverage not only confined to isolated cell protein but also to the ensuing microbial biomass derived from either uni-and multicellular organism, yeasts, filamentous fungi or al- gae that may be employed extensively both as ‘food’ or ‘feed additives’. It has been duly established that the usual protein content (average) in the microbial biomass varies between 45-55%, but in certain specific organism it may be as high as 80%. Because of the presence several essential nutrients ‘biomass’ has gainfully been exploited as an ‘ideal’ supplement to conventional food products across the globe.

Interestingly, in overall comparison with the traditional means and ways of generating proteins both for food or feed, commercial production of microbial biomass affords the following cardinal ad- vantages, such as :

(a) Generally the microorganisms exhibit an exceptionally high rate of multiplication, (b) Microbes possess a high-protein content,

(c) Capable of using a plethora of different C-sources, a few of them could even be waste- products,

(d) Strains with predominantly high yield and markedly excellent composition may be either selected or produced quite easily and conveniently, and

(e) Production of microbial biomass is absolutely independent of either seasonal and climatic variation.

The Central Food Technology Research Institute (CFTRI) at Mysore (India) carried out an inten- sive and extensive research on the utilization of blue-green-algae obtained from the marine sources, Spurulina, as a prominent supplement to diet in elderly and convalescent subjects. The marine-algae is duly cultured, and carefully dried, powdered and used either as tablet (1g) or capsule (1g). It essentially

comprises of protein (upto 60%), essential vitamins and certain unsaturated fatty acids. Spurulina is being used largely as a supplement to the diet of humans both in India and abroad as well.

Disadvantages of Microbial Biomass. The three most vital disadvantages of microbial biomass are as stated below :

(1) A good number of microorganisms give rise to serious toxic products ; and, therefore, one has to ascertain that the biomass is absolutely devoid of such substances.

(2) Microorganisms invariably present in the biomass may give rise to unavoidable indigestion and allergic reaction(s) after consumption.

(3) The excessive high level of nucleic acids (DNA and RNA) present in the microbial biomass products is extremely undesirable.

The SCP Phenomenon. In actual practice, the production of SCP exclusively involves the fol- lowing four major steps that are precisely independent either upon the specific kind of substrate or type of microorganism being employed, namely :

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(1) Preparation of an appropriate medium by making use of various suitable carbon sources e.g., carbohydrates, starch, corsteep liquor, glucose etc.,

(2) Very stringent and effective control, management and prevention of contamination of me- dium and also the entire plant system,

(3) Meticulous production of the desired microorganism essentially required for the fermentative procedure, and

(4) Recovery of the generated microbial biomass and its subsequent processing stages involved. Salient Features : The various salient features with regard to the production of SCPs are de-

scribed as given below : (1) Medium for SCP Production. Importantly, the medium for SCP production varies as per

the type of microorganisms being used.

Examples :

(a) Green Algae (Chlorella, Scenedesmus, Spirulina) — may be cultivated autotrophically* without making use of a dissolved carbon source.

(b) Heterotrophic organisms** (humans) — may be grown heterophically using C-sources derived from two distinct categories, namely :

(i) Fossil : viz., n-alkanes, methanol, ethanol, gaseous hydrocarbons and the like ; and

(ii) Renewable : via., molasses, CO 2 , whey, solid substrates polysaccharide hydrolysates, and effluents of various industries like — distilleries, breweries, potato and canning industries, confectionary industries, and wood-pulp industries.

n-Alkanes : Serve as the most preferred C-source for a wide spectrum of industrial requirements. Source of N : Ammonia gas (NH 3 ) for Saccharomycopsis lipolytica Salts : e.g., Fe 2+ , Mn 2+ ,

Mg 2+ , PO 3– 4 etc. Fungus and Higher Fungi : Mushrooms i.e., fruiting bodies of higher fungi, has proved to be

an excellent delicacy for foods meant for human consumption in a host of countries across the globe

which has almost touched the mark of 10 6 MT each year.

Example : Chaetomium cellulyticum (a fungus) has been gainfully and successfully cultured on

a variety of solid substrates e.g., sawdust, straw, wood chips, composted straw, agricultural wastes, and forestry wastes.

Precautionary Measures : It is, however, pertinent to state here that specifically in the biomass

production the following precautionary measures must be taken religiously, such as : • Culture medium and the entire plant (fermentor etc.) should be free from any possible con-

tamination. • Gaseous components of the culture medium viz., CO 2 , NH 3 etc, and the circulating com-

pressed air must be sterilized by allowing it to pass through filters. • Various other components in several instances are duly sterilized by live-steam.

* Self-nourishing i.e., capable of growing in the absence of organic compounds. ** An organism e.g., a human, requiring complex organic food in order to grow and develop.

MICROBIAL TRANSFORMATIONS

(2) Microorganism : The microorganism being used is suitably cultured in the respective culture medium in perfect clean and hygenic environmental conditions. However, the microorganisms which is meant to be cultured in the medium should essentially possess the under mentioned basic characteristic features, for instance :

(a) Must be non-pathogenic in nature to humans, animals, and plants, (b) Essentially possess excellent nutritional value,

(c) Must be devoid of any ‘toxic’ component whatsoever, (d) Should be largely exploitable as food or feed, and

(e) Overall ‘production cost’ must be reasonably low to render it commercially viable. (3) Choice of Fermentor : In actual practice, the choice of fermentor i.e., the cultivation

vessel or bioreactor, solely depends upon the exact microorganisms to be employed for the operation. Generally, aeration is regarded to be a vital and important functional operation the entire cultivation phenomenon. Heat is invariably produced during cultivation due to the

evolution of CO 2 ; and, therefore, it has got to be dissipated by using a cooling device effec- tively (cooling coils or heat exchangers).

Fig. 4.5 illustrates the outline of an industrial fermentor employed by Imperial Chemical Industry (ICI) to commercially produce SCP starting from Pseudomonas methylotrophus grown carefully upon methanol. By the help of the aforesaid fermentor it may be quite feasible to produce a definite quantum of microbial biomass ranging between 4 to 30 g. L –1 at 38-40°C and at pH 6.8. Importantly, the production is invariably allowed to continue for an indefinite duration to accomplish the maximum achievable targetted economy.

NUTRIENT SOLUTION

SURFACE

DOWNWARD PRODUCT REMOVAL FLOW FOR

COOLING COOLING

AIR-LIFT COLUMN JACKET

NUTRIENT COMPRESSED AIR SOLUTION

Fig. 4.5. ICI-Fermentor to Produce SCP from Methanol.

(4) Harvesting of Microbial Biomass : The various steps that are usually followed for the

harvesting of microbial biomass are as described below in a sequential manner :

(i) Bacteria and yeast i.e., single-cell organisms are invariably recovered by adopting one of these methods, namely : flocculation, floatation and centrifugation.

(ii) Filamentous bacteria e.g., Penicillium notatum are mostly recovered by simple filtration ; (iii) Maximum amount of ‘water’ needs to be discarded before proceeding to the final

stage of drying.

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(iv) Adequate precautionary measures are absolutely mandatory to perform the entire op- eration under utmost clean, hygenic and sterilized conditions to maintain the product and the broth which eventually gets transferred from the plant absolutely free of bac- terial contamination.

(v) The final dried products are found to be fairly stable bacteriologically only if they

are treated and handled as per the stringent operational directives. (vi) Microbial biomass, in certain instances only, essentially require an elaborated and

well-defined after-treatment either in order to minimise drastically the unwanted com- ponents in the said product or to isolate the respective protein(s).

(vii) Reduction of nucleic acids (DNA and RNA) are required to be accomplished on top priority because they may prove to be quite hazardous to health, viz., fungi (2.5 to 6%) ; microorganisms (10 to 16%) ; algae (4 to 6%) ; and yeasts (6 to 10%). However, the various means and ways that are usually engaged in the removal of nucleic acids from the microbial biomass normally include : activation of endogenous nucleases dur-

ing the last-stage of microbial biomass production ; chemical extraction proce- dures ; and ultimately the alkaline hydrolysis.